1. Field of the Invention
The present invention relates to the field of communications and more specifically to the attenuation of an unwanted interferer signal from a communication signal.
2. Brief Description of the Prior Art:
In a cellular telephone system, CDMA (Code Division Multiple Access), including IS-95 and WCDMA (Wideband Code Division Multiple Access), offers an inherent protection against narrowband interference. CDMA relies on a despreading operation to minimise the interference effects of undesired signals, including noise. However, this protection has a cost since it often requires a higher transmission power in order to provide the receiver with sufficient gain over the interference, i.e. to provide the required minimum signal-to-noise ratio. This requirement for higher transmission power reduces the range and the capacity of the network. On the uplink side, this also reduces the mobile station battery duration.
In an article entitled “CDMA-IC: A NOVEL CODE DIVISION MULTIPLE ACCESS SCHEME BASED ON INTERFERENCE CANCELLATION”, and published by IEEE (Institute of Electrical and Electronics Engineers) in 1992, Paul DENT, Björn GUDMUNDSON and Magnus EWERBRING disclose a method for reducing interference in the CDMA signal of one user caused by the CDMA signals of the other users. For that purpose, the user received at higher signal strength is first identified and despread, and this despread signal is subtracted from the composite signal. Thus, for other users' signals not yet despread, interference is made less significant, and their signal-to-noise ratio is improved. Obviously, this method attenuates only interference related to other CDMA users' signals.
In PCT international patent application published under No. WO 99/38270 on Jul. 29, 1999, Terence WIDDOWSON proposes an apparatus for decoding a spread spectrum signal which has been transmitted via a transmission channel. This spread spectrum signal comprises a wanted spread spectrum signal component and an unwanted narrowband signal component. The narrowband signal has been coded using a coding scheme with error correction capabilities. This apparatus includes a narrowband signal substractor in which:                the composite signal is decoded and corrected according to the narrowband signal coding scheme to provide a decoded version of the narrowband signal;        the amplitude and phase characteristics of the transmission channel are estimated;        the decoded corrected signal is encoded according to the narrowband signal coding scheme to produce an estimated narrowband signal;        the phase and amplitude of the estimated narrowband signal are adjusted according to the estimate amplitude an phase characteristics; and        the adjusted signal is subtracted from the received composite signal to provide an estimate of the spread spectrum signal component.        
The narrowband coding scheme can also detect uncorrectable errors. Upon detection of an uncorrectable error, the above mentioned subtraction is suppressed and the frequencies of the composite signal corresponding to the narrowband signal component are attenuated.
Therefore, the apparatus of PCT international patent application WO 99/38270 requires:                decoding and correction of the composite signal;        estimation of the amplitude and phase characteristics of the transmission channel;        encoding of the decoded corrected signal according to the narrowband signal coding scheme;to produce an estimated narrowband signal.        
Document EP 0 967 734 A2 (Suzuki) published on Dec. 29, 1999 relates to a DS-CDMA multiuser interference canceller for processing a received signal containing CDMA spread signals from a plurality of users. The function of this canceller is to remove, from the signal of each user, signals of other users and thereafter decode the signal of this specific user. This obviously eliminates interference caused by the spread signal of the other users.
For that purpose, interference replica signals identical to signal components of the users are generated from a received composite signal and, for every user, the interference replica signals from the other users are subtracted from the received composite signal.
When subtraction of the interference replica signals from the composite signal does not improve reception quality, outputting of these interference replica signals is turned off. Reception quality may be monitored through BER measurement.
Alternatively, a control process can be used to control the level of the interference replica signals, rather than controlling turning on and off of the interference cancellation process.
Again, processing of the received composite signal is required to obtain the interference replica signals. Also, adjustment of the level of the interference replica signal is made according to a predetermined pattern. More specifically, according to the teaching of this document, the level of the interference replical signal is increased with time so that the interference cancellation process is less susceptible to the effect of the delay in a control process.